The relation of dimer size, double-stranded, circular DNA of the replicative form of the bacterial viruses phiX174, S13 and G4 to genetic recombination is being investigated in continuation of our previous work on this subject. A dimeric intermediate, the "figure-8", has been identified by electron microscopy that consists of one dimer-length and two monomer-length circular strands in the form of a covalently-closed duplex. The figure-8 is a configuration in which branch migration can occur. This process has been shown to be obligatory for recombination in the phiX-G4 system. The properties of the figure-8 are being studied by biochemical, genetic and physical methods including electron microscopy and analytical ultracentrifugation. These studies will make use of isolated figure-8s and their infectivity in spheroplast tests of recombination. They will attempt to define the in vivo role of the figure-8 in recombination and its conversion or other relation to circular and catanated dimers. A configuration of DNA derived from the figure-8 of phage G4 by treatment with the Eco R1 restriction enzyme has made available a DNA in which the kinetics of branch migration are being measured and factors evaluated that determine the rate and significance of the process in vivo. Measurements are being made of the rate of renaturation of denatured, covalently closed, circular, double-stranded DNA. Current work emphasizes determination of the dependence of the rate on the molecular weight of the DNA.